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Preheat the oven to 350 F. Line muffin pan with cupcake liners.
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In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
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In a separate bowl, whisk together the egg, banana, water, milk, oil, and vanilla.
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Add the egg mixture to the flour mixture, stirring until well-combined.
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Scoop the batter into the prepared muffin pan, filling it about ¾ full.
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Bake the cupcakes for 20 minutes or until a toothpick inserted in the center comes out clean.
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Transfer the cupcakes to a wire cooling rack and let cool completely.